Boron nitride has been made in the form of soft low-density crystals having a hexagonal crystalline configuration similar to graphite. It has also been formed into very hard high-density crystals with a cubic crystalline formation (hereinafter ZBN) having an atomic crystal structure similar to the mineral zincblende or a hexagonal crystalline configuration (hereinafter WBN), similar to the mineral wurtzite. High-density material having a zincblende structure was disclosed and claimed in Wentorf U.S. Pat. No. 2,947,617. High-density material having a wurtzite structure was disclosed and claimed in Bundy et al. U.S. Pat. No. 3,212,851. The present invention is converned with the preparation of soft low-density crystals having a larger particle size than was previously available. The soft low-density form of boron nitride similar to graphite will hereafter be referred to as "graphitic boron nitride" (GBN).
Most of the processes for preparing GBN utilize boric anhydride as the boron source material. This is reacted with various nitrogen-containing compounds such as NH.sub.3, CaCN.sub.2, or NaCN usually in the presence of an inorganic filler at temperatures varying from 800.degree. C. to 1700.degree. C. Examples of the preparation of GBN are British patent 742,326; U.S. Pat. Nos. 2,808,314, 2,855,316, 2,888,325, 2,839,366, 2,865,715, and 2,834,650; and Russian patent 129,627. The commercially available GBN produced according to any of these processes has particle sizes ranging up to a maximum of one to two microns. U.S. Pat. No. 3,144,305 discloses a process for recrystallization of GBN by heating boron nitride in an inorganic liquid to a temperature above 1000.degree. C. in contact with a preformed hexagonal boron nitride seed crystals. This patent reported recrystallized GBN particles having maximum dimensions up to forty microns,
Pyrolytic boron nitride is a form of GBN produced from the gaseous reaction: EQU 4 NH.sub.3 + BCl.sub.3 .sup.100.sup..degree.-1900.sup..degree. C. BN + 3NH.sub.4 Cl
Pyrolytic boron nitride formed by this process is deposited in well oriented form but with poor crystallinity. U.S. Pat. No. 3,578,408 describes the recrystallization of pyrolytic boron nitide by compression annealing to produce a highly oriented well crystallized material. However, material produced by this process is rather costly in view of the cost of the raw materials used. In addition, the method requires operating temperatures in the 2250.degree. C. - 2,540.degree. C. range.